This paper not to be cited without prior reference to the author ,.. International Council for the C.M. 1964 Exploration of thc Sea Shellfish Committce No. 103 n MYTILICOLA INTESTINALIS STEUER IN DANISH WATERS by Bent Friis Theisen In January 1964 M tilicola intestinalis waD found for the first time in Denmark in musseIs (Mytilus edulis L. from Visby Bredning in the Limfjord (Theisal 1964). SampIes of musseIs (50 spocimens when possible) from a number of localit~s in Danish waters have been examined for Mytilicola. The sampIes were preserved in a 4 per cent formalin solution und oxamined in thc laboratory. The investigation is still in proeess and only mussels from parts of the Danish waters have so far been examined. However, as the distribution of Mytilico­ e la in Danish waters has not previously been rccorded, and as the rather few sampIes examined until now seem to give a clear picture of the distribution I have foundit appropriate to give the results obtained until now. • The distribution and the pereentages of musseIs infested are shown in Figs. 1 and 2. Table 1 is a list of the samplos with dates of sampling, number of musseIs examinod, poreontages of musseIs infested, and average number of Mytilieola por musseL All sampIes from tho western part of the Limfjord were infestod. The poreontagos of infested musseIs in these samplos ranged from 11 to 97 with an aver­ ago number of 0.1 to 19.2 Mytilicola per one musseI. In spita of the high porcon­ tages the infestation seems to bo of a rather recent date. In nearly all the high­ ly infested Dussels was a break in the contours of tho shello (dorsally or ventally viewed)indicating the size of the mUDsel whcn it had become affeeted to such a degree by thc parasite that thc growth-rata had decreaocd. In all eases this brook was found near the margin of the ohell, and none of these musseIs had grown more than a fow millimetres after boing highly infested. Even if the growth of highly infested musseIs is slow thera i8 no doubt that the high infestation only appearcd in thc previous year (1963), and i t is most likely that Hytilieola has lived in thc • Limfjord for a few yearo only. In this conncction it is intoresting that Hancock 4t aD late as 1962 (Hancock 1962) has oxamined 83 musseIs exported from tho Limfjord to England without obsorving any Mytilicola. Today it would be almost impossible to find an uninfosted sampIe from thc exploited musselbeds in the Limfjord. Mytilicola is only found in the west~rn part of the Limfjord. Although fcw musseIs only from the Belt Sea have becn oxaminod the absence of Mytilicola in all sampIes from the eantornpart of the Limfjord, Mariager Fjord, and Randers Fjord indieates that thc parasite has not spread from thc Limfjord to other parts of tho Danish waters. Special attention was paid to the mussclbods in the Wadden­ sea beeause about 2000 t of musseIs had beon transplantod from Thisted Brodning in the Limfjord to that area in May-July 1963. No Mytilicola waro found in tho Danish Waddonsoa neither in the munsels brought thero from the Limfjord nor in the native musseIs. Besidos tho Waddensaa-samplos listod in Table 1 several hundreds of ~us­ sols have been examinod in the field during four viDits to the area in 1964. Ob­ viously tho transplantcd mussels had not beon infested. The apparently rather new infestation in tho Limfjord and the rolatively low infostation in Thisted Bredning (SampIes 6-8) makes it probable that Thistcd Brcdning onlybecamcinfested after the musseIs were transplanted to tho Waddensea. Roforencos Hancack, H. A. 1962 "Starfioh and musseIs" ICES Shellfish Committee No. 83~ Theisen, B. F. 1964 "Mytilicola intestinalis Steuer, en parasitink Copepod ny far den dan8ke fauna~' Flora og Fauna 7°, 2. ~-. - 2 - Tab1e 1. The incidence of infestation of musse1s by Mytilico1a intestina1is in various 1oca1ities. No. of %of Hean No. of Samp1e Loca1ity Date of musseIs Size range musseIs Hytilico1a no. samp1ing examined of musse1s infested per musse1 1 Nissum Bredning 12-6-64 26 45 - 84 92 19.2 2 Ager0 7-2-64 13 56 - 71 38 2.2 3 Visby Bredning 14-1-64 49 40 - 65 96 4.6 4 Skyumore 7-2-64 45 42 - 70 18 0.2 5 Dragstrup Vig 7-2-64 40 47 - 63 15 0.2 6 Thisted 4-2-64 50 46 - 68 16 6.3 7 Hank1it 19-2-64 45 45 - 69 16 0.2 8 na11e Hage 4-2-64 50 47 - 72 26 0.9 9 Harrevig 17-2-64 21 50 - 69 43 1.1 e 10 Lundo Hage 13-2-64 62 29 - 71 11 0.1 e 11 Knudshoved, Fur 7-2-64 29 48 - 74 97 12.5 12 Lundsgard Hage 7-2-64 28 48 - 73 82 12.1 13 Livo 7-2-64 30 28 - 83 77 2.6 14 R0nbjerg Hage 7-2-64 39 42 - 72 41 3.0 15 Bjornsholm 17-2-64 35 31 - 77 80 3.6 16 Nibe Bredning 15-6-64 50 53 - 96 ° ° 17 Bredhage 15-6-64 50 47 - 74 ° ° 18 Hals 5-6-64 50 31 - 74 ° ° 19 Vejdyb 5-6-64 50 33 - 54 ° ° 20 Mariager Fjord 4-6-64 50 38 - 60 ° ° 21 Mariager Fjord 4-6-64 50 50 - 74 ° ° e 22 Mariager Fjord 4-6-64 50 39 - 80 ° ° 23 Randers Fjord 4-6-64 50 39 - 61 ° ° 24 Randers Fjord 4-6-64 50 40 - 58 ° ° • 25 Randers Fjord 4-6-64 50 40 - 60 ° ° 26 Randers Fjord 4-6-64 50 37 - 58 ° ° 27 Tempelkro{; 23-4-64 50 54 - 76 ° ° 28 Horsens Fjord 4-9-64 50 45 - 83 ° ° 29 San~vig 10-8-64 50 31 - 62 ° ° 30 Hojer Dyb+ 2-9-64 50 !r 2 - 65 ° ° 31 H0jer Dyb+ 2-9-64 50 47 - 69 ° ° 32 Hojer Dyb 2-9-64 50 49 - 74 ° ° 33 Hojer Dyb 2-9-64 50 44 - 67 ° ° + ~lus8e1s transpIanted from the Limfjord to the Waddensea. - 3 - e e - e• Fig 1. Distribution of Myti1ico1a intestina1is in Danish waters 1964. The proportion of the circ1e fi11ed in indicates the percentage of musse1s infested. ee Fig. 2. Distribution of Mytilicola intestinalis in the Limfjord, the Mariager Fjord, and the Randers Fjord. The proportion of the circle filled in indicates the percentage of mussels infested..